Alkynes are hydrocarbons that contain one or more triple bonds per molecule. Their names are parallel to the names of the alkanes except that the family ending is -yne. Thus, the four smallest-molecule alkynes are ethyne (more usually called acetylene), propyne, butyne, and pentyne. Alkynes containing one triple bond have chemical formulas given by CnH2n-2, where n is the number of carbon atoms in the molecule. Thus, the formula for pentyne is C5H8. Acetylene, propyne, and butyne are gases at room temperature; the rest are liquids.
The most famous of the alkynes is the first member of the series: acetylene, C2H2. It forms explosive mixtures with air or oxygen, but when mixed with oxygen in a controlled way in an oxyacetylene torch it burns with a very hot flame—up to 6,332°F (3,500°C) which is hot enough to cut and weld steel. Because acetylene is explosive when compressed into liquid form, the tanks of acetylene that welders use contain acetylene dissolved in acetone.
Other important alkenes are styrene, C6H5-CH=CH2, from which the plastic polystyrene is made, and isoprene, CH2=C(CH3)CH=CH2, which is the monomer of natural rubber. (In this shorthand structural formula for isoprene, the parentheses indicate that the CH3 group within them is a branch attached to the preceding carbon atom.)